Studies of past earthquakes have indicated that major loss of life often occurs due to collapse of poorly constructed housing. Conventional earthquake-resistant design of structures has been substantially improved over the last few decades. However, in many countries, the engineering services including design, construction, and inspection allocated to small and low rise private buildings and other structures may not be sufficiently comprehensive to provide an adequate level of structural safety against strong or even moderate earthquake events. If the level of seismic demand, imposed on these buildings, was reduced through a simple but reliable base isolation technique, this would result in fewer building failures and less loss of life.
Steel Reinforced Elastomeric Isolator (SREI) bearings are currently the most widely used isolators. However, as a result of being heavy and high priced, their application has generally been limited to large and expensive structures. Fiber Reinforced Elastomeric Isolators (FREIs) employ fibers rather than steel plates, as the reinforcement sheet. FREI bearings can provide adequate levels of vertical and horizontal stiffnesses as required in a base isolation device. Furthermore, unique aspects such as potentially low manufacturing cost, higher energy dissipation capability, light-weight, and, the possibility of being produced in long rectangular strips and being cut to size as required, provides promising advantages for this type of bearing.
Kelly (1999, Analysis of Fiber-Reinforced Elastomeric Isolators, Journal of Seismology and Earthquake Engineering (JSEE), 2 (1), 19-34) conducted an experimental study on cylindrical handmade bearings consisting of high damped rubber reinforced with Kevlar™ fibers. From the test results, it was revealed that fiber reinforcement can provide acceptable compression stiffness. Additionally, lack of flexural rigidity of reinforcement was shown to have a small effect on the horizontal stiffness of the bearing. The generated hysteresis loops under combined compression and shear showed the same general characteristics as a traditional SREI bearing with a stable behavior up to a peak shear strain of 150%. Furthermore, damping ratios higher than anticipated were obtained which revealed a new source of energy dissipation. This was an unexpected advantage of using fiber as reinforcement in elastomeric bearings.
Seven rectangular carbon-FREIs were tested by Kelly (2002, Seismic Isolation Systems for Developing Countries, Earthquake Spectra, 18 (3), 385-406) under both a compression load to measure the compression stiffness and a combination of compression and shear loading to measure the horizontal stiffness and effective damping. For the latter case, the test was repeated for orientations of 0, 90, and 45 degrees with respect to the longitudinal direction of the strip. It was observed that loading along 0 degree produces stiffening in the hysteresis loops, whereas along 90 degrees (i.e., the cross direction) softening behavior tends to occur. Loading along 45 degree produced neither softening nor stiffening. Experimental results confirmed that it is possible to produce a strip FREI that matches the behavior of a SREI. The measured horizontal stiffnesses and the maximum accommodated displacement indicated that the concept of carbon strip-FREI is viable.
Moon et al. (2003, Mechanical Property Analysis and Design of Shock Absorber System Using Fiber Bearing by Experimental Method, JSME International Journal, 46 (1), 289-296) compared the performance of a cylindrical carbon-FREI to that of the same size SREI. The difference between the steel plate and the fiber thickness was adjusted by using more layers of fiber and rubber in the FREI. Accordingly, bulging of the FREI was smaller than that of the SREI due to the thinner layers of rubber in the FREI. Unlike previous studies by Kelly where the bearings were built without end plates and were not bonded to the test machine during the test, both bearings were bonded to thick end plates. The researchers concluded that the performance of the FREI is superior to that of the SREI. However, due to insufficient information regarding the details of the tested isolators, the basis of comparison is not clearly identified. Accordingly, comparative studies between FREIs and SREIs still need to be conducted.
Summers et al. (2004, Development of new base isolation devices for application at refineries and petrochemical facilities, 13th World Conference on Earthquake Engineering Vancouver, B.C., Canada, August 1-6, Paper No. 1036) conducted an experimental study on prototype rectangular carbon FREIs as a potential seismic protection strategy for liquid storage tanks. The bearings consisted of high damped rubber compound and were subjected to a maximum 100% shear deformation under constant vertical compression. The resulting hysteresis loops showed stable behavior.
FREI bearing or isolation pads are used in both new and retrofit construction to provide vibration damping for residential and commercial properties and prevent structural collapse during seismic events. Various FREI bearings are disclosed in prior art issued patents and published patent applications.
US Patent Publication No. 2004/0123530 describes a system for protecting a structure from seismic ground motion comprising a horizontal bearing surface and damping elements consisting of unreinforced rubber slabs and fiber-reinforced elastomeric mats. Each mat consists of a piece of a rubber mat reinforced with fibers extending parallel to the surfaces of the mat. The horizontal bearing surface is covered with several layers of the damping elements.
U.S. Pat. No. 5,014,474 describes an apparatus for limiting the effect of vibrations between a structure and its foundation having two types of elastomeric load bearing pads. One pad absorbs vertically and horizontally applied forces. The other pad can accommodate sliding motion between the structure and the foundation and can absorb horizontally applied forces.
U.S. Pat. No. 5,904,010 describes a fiber reinforced elastomeric seismic isolation bearing. The bearing is a laminate block of material with alternating layers of elastomer and woven fiber mesh having fibers 0.1 to 1 mm made of graphite or a polyamide polymer, such as Kevlar™. This patent however describes bearings with vulcanized layers and pretensioned fibers.
U.S. Pat. No. 4,695,169 illustrates a structural bearing made with an elastomeric core imbedded with a matrix of twisted Kevlar™ cord. Rollover is not disclosed in this patent.
U.S. Pat. No. 4,593,502 describes a rectangular energy absorber constructed from layers of resilient materials and stiffening materials. The energy absorber also includes a steel spring and a lead core. The absorber can also be constructed with alternating layers of resilient and stiffener material. The layers described in this patent are not vulcanized and the fibers are not pretensioned. Rollover performance is not disclosed in this patent.
U.S. Pat. No. 4,887,788 describes the general state of the art as a base isolation pad to absorb energy. The disclosed device is made of an elastomeric material with a flexible reinforced tubular restraining shell.
U.S. Pat. No. 5,233,800 describes an earthquake proofing device designed to dampen the effects of seismic activity. The pads of this device incorporate laminated layers of synthetic rubber with specific characteristics and rigid plates made from wire.
U.S. Pat. No. 5,438,806 describes a composition for designing a device for vibration damping of released energy. The device consists of a number of configurations of elastomeric layers containing air or fluid voids.
The bearings of the prior art consider the rollover effect as an undesirable parameter which decreases stability.
There remains a need to provide a base isolation system which is intended for cost effective seismic mitigation of structures such as small and medium low-rise buildings and bridges. Such a system would require FREI bearings in an unbonded application, which demonstrate increased stability with stable rollover deformation.